Cosmetics are blended with an oil-based agent for various purposes of providing skin occlusiveness or emolliency, and improving a textural feeling. An oil-based agent to be incorporated is required to have a favorable usage feeling, safety, compatibility with other raw materials, and the like. Among these, a polyglycerol fatty acid ester, which is an oil-based agent, exhibits high safety and is therefore generally used as a surfactant and an emulsifier.
Polyglycerol is a general term referring to glycerol polymers, and a representative structure thereof is a linear polymer resulting from dehydration condensation of primary hydroxyl groups of glycerol. Further, polyglycerol embraces, in addition to the linear polymer represented by the following formula (1), a branched polymer resulting from dehydration condensation between primary hydroxyl groups and secondary hydroxyl groups of glycerol, and a cyclic polymer resulting from dehydration condensation between intramolecular hydroxyl groups. Among these three polymer species, a linear polymer is a predominant component, but the content ratio therebetween may vary depending on the production method, the degree of polymerization, or other factors.

n: degree of polymerization
Industrially, polyglycerol, which is obtained by a dehydration condensation reaction of glycerol or by recovery of polyglycerol from glycerol distillation residues, has been used in foods or the like, whereas high-purity polyglycerol products, derived from epichlorohydrin, glycidol or the like, have been limited to applications for assay reagents or the like. Further, in the case of polyglycerol from recovery thereof from glycerol distillation residues, such recovery is mainly intended for the production of diglycerol and correspondingly the majority of polyglycerol has been produced through a
dehydration condensation reaction of glycerol.
The dehydration condensation reaction of glycerol is a sequential intermolecular dehydration condensation reaction, and polyglycerol obtained by such a reaction exhibits various polymerization degrees and isomeric ratios depending on the reaction conditions. Therefore, the polymerization degree of polyglycerol is generally given in terms of an average degree of polymerization calculated from a measured value of a hydroxyl value.
Polyglycerol for practical use is provided in the form of such a complex mixture. For example, polyglycerol generally referred to as hexaglycerol encompasses monomers and polymers ranging from monomeric glycerol to nonaglycerol having a degree of polymerization of 9, and in terms of polymers thereof, covers isomers as described above.
Since it is generally known that a polyglycerol fatty acid ester exhibits high stickiness, and when blended as an oil-based agent in a large amount in a cosmetic, contributes to deterioration of performance in a usage sensation and texture of cosmetics, a variety of improvements have been attempted therefor (for example, see Patent Document 1).
Further, an oily phase and an aqueous phase are blended into emulsion cosmetics, but aggregation or creaming may take place due to a difference in a specific gravity therebetween. For this reason, it is necessary for a cosmetic to have an appropriate viscosity in order to secure an emulsion stability.
Conventional lotions and emulsions have been used separately from one another for achieving a stability of a preparation. However, recently, from the viewpoint of convenience of use or the like, there is a need for a novel cosmetic which is capable of achieving a usage sensation of both a lotion and an emulsion by means of a single cosmetic. Such a novel cosmetic is required to have fresh texture at a low viscosity due to exhibiting a relatively high moisture content. However, as described above, there is a need in related art for an appropriate viscosity to obtain a cosmetic with a high emulsion stability, and a cosmetic having a high emulsion stability even at a low viscosity (for example, see Patent Documents 2 and 3).
A self-emulsification-type oily liquid cosmetic, when brought into contact with water, undergoes a phase inversion to form an oil-in-water emulsion. Representative examples of the self-emulsification type oily liquid cosmetic include a cleansing cosmetic and a bath cosmetic. The bath cosmetic has been frequently used in bathing for the purpose of preventing rough skin, chapped skin and cracked skin or improving the skin condition. Examples of the bath cosmetic formulation include a bath salt, a bath oil, and a crude drug extract. Among these, a self-emulsification-type oily bath oil type product consisting of a mixed system of an oily component and a surfactant, when put into a bath, undergoes self-emulsification in the bath to result in uniform distribution of a particulate oily component in the bath, whereby a variety of beneficial effects intrinsic to the oily component may be expected such as emolliency, a moisture-retaining property, an anti-inflammatory property, and a warm bath effect. However, products of a self-emulsification-type oily bath oil-type containing a polyglycerol fatty acid ester have a problem associated with a deteriorated usage sensation due to creaming or separation of an oily component after being put into hot water. An attempt has been made to cope with this problem (for example, see Patent Document 4).
Further, as the formulation of a bath agent, there is an emulsion bath agent which is characterized in that an oil-soluble substance and a water-soluble substance may be blended and added without limitation. However, an emulsion bath agent is required to have a low viscosity in terms of a usage characteristic, and therefore has a problem associated with a poor stability over time of the bath additive itself. Therefore, an attempt has been made to address this problem (for example, Patent Documents 5 and 6).
Further, with regard to a two-separate-layer-type cosmetic, there is a need for development of a two-separate-layer-type cosmetic which includes an oily layer and an aqueous layer and exhibits an excellent aesthetic appearance in terms of external appearance, and which can be easily dispersed uniformly by gently shaking before using and, after using, easily separated into the original two layers of an oily layer and an aqueous layer, and remain stable. To this end, a variety of suggestions have been made (for example, see Patent Document 7).